A lot of types of High sensitive micro sized magnetometers have been developed since 1936 which consist of magnetic wire types named as Parallel-gated Flux gate sensor, Orthogonal-gated Flux Gate sensor, MI sensor, Coil type MI sensor, and high frequency carrier type sensor based on magnetic wire technology and magnetic thin film types named as Hall sensor, MR sensor, and tMR sensor based on semiconductor technology.
At present many types of the electronics compass based on these sensors except high frequency carrier type sensor are widely used for navigation service in smartphone and automobile. High performance types based on Parallel-gated Flux gate sensor, Orthogonal-gated Flux Gate sensor, MI sensor, and Coil type MI sensor are developed. Low cost types based on Hall sensor, MR sensor, and tMR sensor are developed. In future the compass with higher performance and lower cost will be expected to be used as a motion input device of wearable computer.
Thin film types named as hall sensor, GMR senor, and tMR sensor are advantage in cost and mass production but not so good in sensitivity, heat resistance and current consumption. The present inventor made efforts to invent the ultra-high sensitive micro sized magnetometer based on new technology combined with magnetic wire technology and semiconductor technology.
Parallel-gated Flux gate sensor developed in 1936 has been achieved to detect a static magnetic field of nT (nano tesla) level by the means of the pickup coil to measure the amplitude of the second harmonics of the incremental deference of two magnetic wire magnetizations which were respectively magnetized by one right turn coil and other left turn coil to pass through AC current of 30 KHz.
Orthogonal-gated Flux sensor hereafter called as FG sensor, developed in 1952 referred as patent literature 1 has achieved smaller size of the wire length of 50 mm than that of Parallel-gated Flux gate sensor by the means of the coil binding the wire to detect the rotation of the longitudinal magnetization of the wire excited by AC current of 30 KHz.
The FG sensor excited by pulse current with frequency of 500 KHz was invented in 1988 referred as patent literature 2 to improve sensitivity with a magnetic amorphous wire with length of 50 mm and resistivity of 100 μΩcm. The magnetic property of the amorphous wire with the diameter of 125 μm used had permeability of over 3000 same to permalloy used in FG sensor. The magnetic domain structure of both alloy consisted of longitudinal domains formed by 180 degree walls.
The principle of FG sensor is to measure the rotation of longitudinal magnetization of the wire vibrated by the movement of the 180 degree magnetic wall forced by the circular magnetic field produced by AC current or Pulse current. The longitudinal magnetization of the wire is lowered by the diamagnetic field inverse proportional to the wire length. It is difficult to decrease the wire length without the decrease of the sensor sensitivity, which means that a ultra-high sensitive micro sized magnetometer cannot be developed based on FG sensor technology.
MI sensor referred in patent literature 3 was developed in 1993 on the base of magneto impedance effect called as MI effect. The magneto impedance effect appears on passing the AC current with frequency of 10 MHz to 100 MHz through a magnetic amorphous wire.
The magneto-impedance of the wire is drastically increased dependent on the external magnetic field due to the skin effect induced by high frequency current. The external magnetic field can be detected from the relationship between the external magnetic field and the impedance change. The influence of the frequency on the magneto-impedance shows it has a maximum value at 10 MHz and decreases beyond 10 MHz due to eddy current increased proportional to frequency.
However, MI sensor had some drawback to give the poor properties in non-linearity and not negligible hysteresis. These problems could be solved using a negative feedback circuit and a bias coil which were accompanied with big current consumption. The details on MI sensor technology has been explained in the Non patent literature 1 named as “the theory and technology on the magnetometer” written by Kaneo Mohri published by CORONA PUBLISHING CO.LTD in 1998.
A coil type of MI sensor referred in patent literature 4 was developed in 1999 to achieve linear output detected by the coil surrounding the enameled wire around the magnetic amorphous wire.
The magneto-impedance effect of the magnetic amorphous wire is caused by passing the AC current with the frequency of 10 MHz. This wire has the special magnetic domain structure consisting of the core domain with the longitudinal spin alignment, the surface domain with the circular spin alignment and 90 degree magnetic wall between both domains. When the AC current is passed through the wire, the magnetic wall is vibrated from the surface to the inner side alternately, accompanied with vibration of magnetization rotation. The magnetization rotation is detected by the coil surrounding the wire as its voltage.
The structure of the coil type of MI sensor is same to that of FG sensor referred as patent literature 2. The 180 degree magnetic walls of the wire can move easily up to around the frequency of 30 KHz due to resisting power of eddy current increased with the frequency. However, MI sensor is caused by movements of the 90 degree magnetic wall between the core domain and the surface domain. The 90 degree magnetic wall can move easily up to around the frequency of 200 MHz because the boundary connected to the surface domain with circular spin alignment is easily moved by circular magnetic field.
The coil voltage increases with f1/2 of the frequency f so that MI sensor operated with 200 MHz could provide about 100 times larger coil voltage than that of FG sensor operated with 30 KHz.
Other advantage of the coil type MI sensor using the wire diameter of 30 μm could be achieved from 50 mm of FG sensor to the short wire length of 3 mm. It is noted both sensors have same structure but are operated by deferent frequency.
However its type used the negative feedback circuit to decease the hysteresis which made serious drawback to increase the current consumption.
Subsequently the plating coil type of MI sensor referred in patent literature 5 was developed in 2004 to decrease the current consumption by omitting a negative feedback circuit. In addition it could provide its small sensor size by the small sized MI element which was produced by plating process on the substrate.
The pulse annealing method instead of a negative feedback circuit is applied to the plating coil type of MI sensor for vanishing the hysteresis or improving the linearity. While the rectangular pulse is applying to the wire, the 90 degree magnetic wall goes from the surface to the wire center to vanish the hysteresis. The coil voltage detected at the moment of pulse falling gives no hysteresis.
The small sized element consisted of the magnetic amorphous wire with the diameter of 12 μm and length of 0.6 mm and the coil with the inner diameter of 30 μm and the pitch of 30 μm.
This type of MI sensor is widely used as the electronics compass named as AMI306 in the mobile phone which has measuring range of ±10 G using the amorphous wire with the anisotropy field of 20 G.
The electric circuit of the coil type MI sensor consists of the Pulse generator, MI element with the coil, the sample holding circuit equipped with an electronics switch and a capacitor, detection timing adjusting circuit, and a programming amplifier. It can detect the holding voltage of the capacitor given by integrating the current induced in the coil during a period which takes coil voltages from zero voltage to the peak voltage. The holding voltage V corresponds to the magnetization M of the wire related to the external magnetic field expressed by the equation of M=χH. The external magnetic field H is calculated from the voltage V within the linear region between the external magnetic field H and the magnetization M. The measuring range keeping the linearity would be about 30% of the maximum external magnetic field measured at the peak voltage.
The MI sensor to measure the magnetic impedance has the optimum frequency of 10 MHz because the magnetic impedance expressed as combination of a real part and imaginable part corresponds to the energy loss against the AC current. Its skin effect increases the real part of the impedance to give the maximum magnetic impedance at the frequency of 10 MHz.
However the coil type MI sensor measures the coil voltage caused by the rotation of the magnetization M. The rotation of the magnetization increases the imaginal part of the impedance to give the maximum coil voltage at the frequency of 200 MHz.
Above mentioned, the high sensitive micro sized magnetometers have progressed in sensitivity and small size based on magnetic wire technology since 1936. The coil voltage is expressed by the equation (1) which shows the tradeoff relationship between sensitivity, element size and measuring range. It can increase by means of the improvement of the wire properties, increase of frequency and micro coil with the coil pitch of under 10 μm.V∝N·f1/2·μ·Hex·D·L   (1)
The performance of sensors can be estimated by the index defined as S=W/KLD where K means sensitivity or noise, LD means volume, and W means measuring range.
Its performance of various sensors were calculated as bellow.
FG sensor operated with frequency of f=30 KHz has index S=0.1 when noise of K=0.2 mG, measuring range of W=2 G, length of L=50 mm, diameter of D=2 mm.
MI sensor operated with frequency of f=10 MHz has index S=60 when noise of K=0.2 mG, measuring range of W=2 G, length of L=5 mm, diameter of D=0.03 mm.
The plating coil type MI sensor operated with frequency of f=200 MHz has index S=800 when noise of K=2 mG, measuring range of W=10 G, length of L=0.6 mm, diameter of D=0.01 mm.
The progress on the performance of those sensors have been caused by increase of the frequency from 30 KHz via 10 MHz to 200 MHz. The epoch making innovation in progress was to discover MI effect caused by vibrating 90 degree wall existing between the core magnetic domain and the surface magnetic domain. The details of the progress of MI sensor has been explained in Non patent literature 2 named as “the new magnetometers and their applications” written by Kaneo Mohri published by Triceps Co. in 2012.
Nowadays these type of high sensitivity micro sized magnetometers are widely used as the electronics compass in smart phones and in future are expected to improve high sensitivity from 2 mG to 0.2 mG and wide measuring range from ±10 G to ±40 G, small size of element from 0.6 mm to 0.2 mm and low current consumption for application to the motion sensor for wearable computers. In other words, the index S of the performance must be improved to 100,000 when K=0.2 mG, W=40 G, L=0.2 mm, D=0.01 mm.
The inventor invented the MI sensor with more high sensitivity driven by high frequency pulse of 0.3 GHz to 1.0 GHz referred in patent literature 6 in 2009 but gave up the development of commercial product because there were a lot of problems such as big noise accompanied with high frequency pulse, increase of the parasitic capacitance of the pickup coil and difficulty to make design of electronic circuit to detect the coil voltage because it is very fast.